The present invention is directed to aberration correction in a scanning (or multiple beam) confocal antenna system.
Confocal antenna systems having main and subreflectors are well known and widely used. Two common types of multiple reflector antenna systems are the Cassegrain reflector system and Gregorian reflector system. FIG. 1 illustrates an offset (or eccentric) confocal paraboloidal antenna system of the Gregorian type. The magnification of the system shown in FIG. 1 is defined as m=f.sub.1 /f.sub.2, where f.sub.1 is the focal length of the main reflector 10 and f.sub.2 is the focal length of the subreflector 12. With such a magnification, in order to form a beam having a propagation direction 14 forming an angle .theta..sub.0 with respect to the system axis 16, the wave front generated from the plane wave feed 18 via the array of radiating elements 20 must be tilted at an angle of approximately m.multidot..theta..sub.0.
The reflector system shown in FIG. 1 has several characteristics which are desirable in a scanning or multiple beam antenna. The system is fully corrected for all orders of spherical aberrations, for third and fifth order coma aberrations, and for third order astigmatism. Further, when the plane wave feed system consists of a phased array, the physical size of the array can be reduced by a factor of m relative to the radiating aperture of the system.
Generally, the antenna designer would like to make m large in order to reduce the size of the feed system. However, this involves a necessary trade-off against field of view requirements, i.e. the required scanning range of the feed, and curvature of field, distortion and higher orders of coma and astigmatism aberrations which are not corrected in the system and are dependent on m. The result is frequently a system in which the physical size of the subreflector approaches that of the main reflector, which is undesirable and impractical for most applications.